Process for reducing corrosion



United States Patent Ohio 1 No Drawing. Filed May 24, 1965, Ser. No.458,436 8 Claims. (Cl. 204-447) This invention relates to improvement inthe cathodic process of protecting ferrous objects from corrosion and tothe preparation of the articles so protected.

The term ferroussurfaces is used in this specification to mean articlesmade of iron, iron alloys andv various types of steel.

The protection of, ferrous articles from corrosion by cathodic methodsis frequently accomplished by using the so-called sacrificial anodeswhich are so made of metals more active than iron. In this process thesacrificial anode forms an electrolytic cell with the ferrous article,and the sacrificial anode is eroded away in preference to the ferroussurface, thus protecting the ferrous surface from corrosion. Cathodicprotection can also be provided by impressing an external current on theferrous article rendering it electronegative with respectto itssurroundings. The current density required to protect a coated articleis dependent on the ohmic drop through the coating and the currentrequired to polarize surface areas of metal exposed by pinholes, flawsor breaks in the coating.

In coated ferrous articles which are cathodically protected the rate ofcreeping of electrolyte under the coating at a damaged area is muchgreater than the rate of creeping of electrolyte under the coating at adamaged area in articles not cathodically protected. The infusion ofelectrolyte causes a loss of adhesion with concomitant disbonding, ofthe coating and an accumulation of water and electrolyte forms under theloosened coating. In these areas metal corrosion can take place andcathodic protection is not sufiicient to prevent it.

According to this invention ferrous articles first treated to provide aphosphate coating on the surface of such articles and then coated with afused resin coating have a reduced rate of disbonding of the coatingwhen the article is exposed to electrolyte solutions under cathodicprotective conditions.

The practice of the invention is illustrated in the following examples:

EXAMPLE 1 A fusion blend of 70 parts by weight of a 70/30 ethyl eneterephthalate-neopentyl terephthalate copolyester resin which had aglass temperature of 72 C. and an intrinsic viscosity of 0.650, 30 partsby weight of a co polyester of ethylene glycol, neopentyl glycol,terephthalic acid and sebacic acid in which the ethylene glycol unitsand neopentyl glycol units were present in the ratio of 80:20 and theterephthalic acid units and sebacic acid units were present in the ratioof 80:20 which had a glass temperature of 23 C. and an intrinsicviscosity of 0.775 and 5 parts of titanium dioxide powder was preparedby mixing in a Banbury type mixing apparatus at a temperature in therange of from room temperature to 135 C. for minutes. It was thendischarged from the mixer and ground to a powder having a particle sizeof less than 40 mesh, US. Standard. The powdered resin compound wasplaced in an Armstrong Model C Vibro-Fluidizer apparatus and theparticles were suspended or fluidized by passing dry air at a rate of 40cubic feet per hour through the layer of particles.

A piece of 1 /2" diameter iron pipe 8 inches long was grit blasted tobright metal and vapor degreased by treatment with trichloroethylene. Itwas then heated in a forced 3,409,525 Patented Nov. 5, 1968 draft ovento 300 C. The hot pipe was removed from the oven and immersed in thefluidized powder compound in the Fluidizer apparatus for 3 seconds,removed from the Fluidizer and allowed to fuse in air for 20 seconds andthen quenched in cold water for about 2 minutes.

The coated pipe was then tested in an accelerated aging test known asthe Salt Crock Test as follows:

A diameter damage hole was drilled through the coating 2 /8" from oneend of this pipe. The end was stoppered, bound with two or three turnsof vinyl electrical tape, dipped in a vinyl patching compound and airdried for 16 hours. The coating was ground off the other or free end ofthe pipe sample and an alligator battery clip was attached making asound electrical connection. The closed end of the pipe sample was thenimmersed in an electrolyte bath along with a one inch diameter graphiterod with a 4 inch space separation between them.

The electrolyte solution contained 0.317 gram of each of the followingsalts: sodium chloride; sodium sulfate; and sodium carbonate (allcalculated on anhydrous basis) per liter of distilled water. The drilleddamage hole was positioned so that it faced the graphite rod. The pipesample was connected to the negative pole of a 6-volt DC power source.The graphite rod was connected to the positive pole of the power source.The power was applied to the pipe and the graphite rod by the powersource over a period of 30 days. The solution level in the bath wasmaintained by the periodic addition of distilled water.

The coating was examined for disbonding around the damage hole 'bymaking parallel cuts through the coat ing about A; inch apart andpeeling a strip of coating back from the edge of the drilled hole infour directions. The distance from the edge of the hole which thecoating can be peeled is measured in each direction and an average valuecalculated.

After the 30 day Salt Crock exposure the coating exhi'bited a disbondingof 1% inches.

EXAMPLE 2 An 8 inch piece of iron pipe of the type used in Example 1 wascleaned in an alkali bath and pickled in a 10 per cent phosphoric acidbath.

The thus treated pipe was then dipped in a phosphating solution made of5 parts of Meta Bond 14 in 95 parts of water maintained at a temperatureof C. for 5 minutes and rinsed with a dilute chromic acid solution. MetaBond 14 is a trade-name for a commercial product of the InternationalRustproof Corporation whose composition is approximately as follows:

The thus treated pipe was then coated with the polyester resin compoundin the same manner described in Example 1. The coated pipe was testedaccording to the procedure used in Example 1. After the 30 day SaltCrock Test the coating exhibited an average disbonding of inch.

EXAMPLE 3 An 8 inch piece of iron pipe of the type used in Example 1 wasgrit blasted to bright metal, cleaned in an alkali solution and thendipped in a phosphating solution maintained at a temperature of 90 C.for 5 minutes as in Example 2 and rinsed with dilute chromic acid. Thethus treated pipe was coated with polyester resin compound by the sameprocedure used in Example 1. The coated pipe was then tested accordingto the procedure used in Example 1. After the 30 day Salt Crock Test thecoating exhibited an average disbonding of inch.

After the metal article has been treated with phosphate, rinsed withchromic acid, and dried it can be stored prior to coating with resin orit canbe coated immediately after it hasbeen treated and dried, ifdesired. The improved reduced: rate of disbonding is still obtainedwhether the coating is applied immediately or after storage.

-Generally, prior to treating with phosphate the meta is cleaned bygritblastingjby vapor degreasing, by alkalicleaning, by pickling, or by.a combination of these methods Whenoil .or grease are on the surface ofthe metal, degreasing is generally performed. i

The invention has beenillustrated particularly with re spect tothe useof a polyester resin blend. Other blends or other polyester resins canbe used. Representative examples of such resins are the polyesters ofterephthalic acid, bibenzoic acid, the naphthalene'dicarboxylic acidswith glycols such as the polymethylene glycols and copolyesters of theseacids with modifying amounts of other acids and other glycols.Especially suitable polyester resins are the blends disclosed inco-pending application Ser. No. 301,598, filed Aug. 8, 1963. Otherresins such as the polyethylenes, polyamides, polystyrene, vinyl resinsand epoxy resins can also be used. The resins may contain plasticizers,pigments, coloring agents and other compounding ingredients. If desired,a primer coat can be applied to the article after the phosphatetreatment and before it is coated with resin. Thus, primer coats ofrubber, plastics, epoxy resin or other types of primers can be applied.

The invention has been particularly described with respect to treatingthe metal to produce a microcrystalline zinc phosphate coating. Otherphosphate treatments can be used. The preferred treatments coat theferrous article with amorphous or microcrystalline phosphate coatingssuch as amorphous zinc phosphate and microcrystalline zinc phosphate.The size of the phosphate crystals can be adjusted by the addition ofmodifiers to the phosphate solutions.

The articles coated may be of any desired shape. The articles may besheets, castings, pipe, rods and the like. Any of the several powdercoating methods can be used to apply the coating of fused resin. Thusthe articles can be coated by the fluidized bed process, theelectrostatic powder coating process, the cyclone process or the flamespray process, and the advantages of the invention obtained.

While certain representative embodiments and details have been shown forthe purpose of illustrating the invention it will be apparent to thoseskilled in this art that various changes and modifications may be madetherein without departing from the spirit or scope of the invehtion.What we claim is:

1. In the cathodic process of protecting ferrous articles from corrosionthe improvement which comprises subjecting a ferrous article to atreatment that forms a phosphate coating on the surface of the article,then coating it with a fused resin, and then cathodically protectingsaid article.

' 2. The process of claim 1 in which the phosphate coating formed is anamorphouseoating of zinc phosphate. 3. The process 'ofclaim 1 which the'resin coating forriiedbn theartic le is a polyester-resincoating." 4.The pi'oces swhich comprises matings: I ticle with va solution thatform's jiine' phosphateeoatlihg on the surface ofthe article, drying.the treated article, heating the article and immersing it in the densephase of a-fiuidizedbed of polyester resin particles. while-the articleis heated to'a temperature at le'astasshigh :as the:

fusion temperature of the resin particles but below the-.der compositiontemperature ofthe: particles-untilva layer 0f resin of desired thicknessis formed on the' article, allow-- ing the resin particles to fuse intoa continuous coating, cooling the coated article, 'and:thencathodicallyprotecting said article; I 1

5. The process of claim 4 in which the zinc'phosphate coating formed isamorphous.

"6. The process of claim '4 in-which the'zinc phosphate coatingiformedis microcrystalline. 7. Inthe cathodic process of protecting ferrousarticles from corrosion the improvementwhich comprises forming aphosphate coating on thesurface of said article, then coating it witha-fused resin coating and then forming an electrolytic cell in which thethus coated article is the cathode. 8. In the cathodic process ofprotecting ferrous articles from corrosion in which a currentis'impressed'on said article to 'make it electronegative with respect toits surrounding environment the improvement which comprises forming aphosphate'coating on the surface of said article, coating it with afused resin coating and then impressing the current on the article tomake it electronegative. 3

References Cited UNITED I STATES PATENTS 2,491,225- 12/1949 Stearns204-196 2,623,031 12/1952 Snyder 260-860 3,001,919 9/1961 Petrocokino204197. 3,097,118 7/1963 Leonard 148-6.,15 3,106,769. 10/1963 Goethe etal. 11721 3,116,178 12/1963 Upham 1486.15 3,272,647 9/1966 Swansonet al.11775 HOWARD S. WILLIAMS, Primary Examiner.

T. TUNG, Assistant Examiner.

1. IN THE CATHODE PROCESS OF PROTECTING FERROU ARTICLES FROM CORROSIONTHE IMPROVEMENT WHICH COMPRISES SUBJECTING A FERROUS ARTICLE TO ATREATMENT THAT FORMS A PHOSPHATE COATING ON THE SURFACE OF THE ARTICLE,THEN COATING IT WITH A FUSED RESIN, AND THEN CATHODICALLY PROTECTINGSAID ARTICLE.